Ukrainian border guards say Moscow has poured almost 20-thousand troops into crisis-hit Crimea.
That is on top of the 11-thousand already permanently based with Russian Black Sea fleet in the port of Sevastopol.
President Vladimir Putin denies that forces with no national insignia are under Moscow’s command. He has also rebuffed a US warning over military intervention.
Following a telephone conversation with Barack Obama, Putin reiterated that his country cannot ignore calls for help from Russian speakers in the region.
Obama announced the first sanctions against Moscow since the start of the crisis.
Meanwhile, unarmed military observers deployed by the Organisation for Security and Cooperation in Europe have been denied access to Crimea – after arriving in the Ukrainian port of Odessa.
The crisis escalated on Thursday, when Crimea’s regional government set a referendum on the peninsula’s status for 16 March.
The vote is coming under from the West, but it has won Russian backing. One Russian parliament leader has stressed that Crimea has a right to self-determination.